Unexpected token 'I', Invalid request body is not valid JSON

Continued working on up my SvelteKit + Supabase Auth Helpers project from yesterday... tried signing in, and... suddenly I'm getting a 500 error when Supabase tries to apply auth actions.
Unexpected token 'I', "Invalid request body" is not valid JSON
SyntaxError: Unexpected token 'I', "Invalid request body" is not valid JSON

?! Only a few minor changes to the project from what I pushed to production yesterday (and which works). Cannot for the life of me see what suddenly changed. This error happens in
npm run dev
and npm run build. Here's the DevTools window:

Any ideas? Where should I look?
Screen_Shot_2022-11-15_at_10.04.49_AM.png
Screen_Shot_2022-11-15_at_11.59.32_AM.png
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